1. Field of the Invention
This invention relates to an method and apparatus for soldering microelectronic lead connections to pads on a ceramic substrate with the aid of a fiber based neodymium:yttrium-aluminum-garnet (Nd:YAG) laser. Such structures of this type, generally, provide a means of delivering sufficient localized heating to the ceramic substrate to reflow solder plate or melt solder cream (paste) within the lead connection without any substrate preheating.
2. Description of the Related Art
It is known, in the microelectronic field, that there are many components that are becoming more and more sensitive to the packaging process. In particular, there are microelectronic chips that are sensitive to excess heating. When ceramic substrates are used for electronic packaging, excess heating can occur during a solder reflow lead connection process leading to potential component damage. This results from the relative high thermal conductivity of aluminum oxide and aluminum nitride as compared to an epoxy-glass substrate. The thermal conductivity comparison ratio would be approximately 364:57:1 for the material ratio of AlN:Al.sub.2 O.sub.3 :epoxy-glass. The thermal conductivity of epoxy-glass is approximately 0.294 Watts/m-C.
Typically, a ceramic substrate is pre-heated to some subreflow temperature for soldering prior to heating just the lead/pad assemblies to the solder reflow temperature. In some cases, the required substrate pre-heating can be performed by conduction heating with a hot plate. Also, vapor phase and infrared techniques could be used. Finally, additional heating of the lead/pad assembly to the reflow temperature can be accomplished with a soldering iron or any single point heat source. A more advantageous system, then, would be presented if such amounts of preheating were reduced or eliminated.
It is apparent from the above that there exists a need in the art for soldering system which is capable of soldering microelectronic lead/pad assemblies to ceramic substrates, and which at least equals the performance characteristics of known soldering systems, but which at the same time substantially eliminates the preheating of the substrate. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.